1. Field of the Invention
The present invention concerns a transducer with metallic electrodes interdigitated in comb form and deposited on a piezoelectric substrate. The transducer is included, particularly, in a surface acoustic wave filter.
In such filters, it is sought to reduce the insertion losses, which are fairly high, to the maximum extent. The interdigital transducers must be significantly mismatched to reduce the effect of the stray echos of the acoustic wave being propagated in the filter where the losses generally exceed 20 dB.
2. Description of the Prior Art
To reduces insertion losses, it is known to make filters with interdigited transducers with conjugated admittances. This type of filter is described in the article by M. HIKITA et al., "HIGH PERFORMANCE SAW FILTERS WITH SEVERAL NEW TECHNOLOGIES FOR CELLULAR RADIO", IEEE Trans. Microwave Theory Tech., vol. 33, No. 6, pages 510 to 517, 1985.
Referring to the appended FIG. 1, which corresponds to FIG. 8a of the above-mentioned article, this type of filter has two parallel input and output parallel acoustic channels VE and VS on a main surface of the substrate. These two channels have several juxtaposed coupling cells in common, for example CE.sub.1 and CE.sub.2 according to the illustrated embodiment. Each cell CE.sub.1, CE.sub.2 includes two interdigital input transducers TE connected to an input terminal BE of the filter, two interdigital output transducers TS connected to an output terminal BS of the filter, and two intermediate interdigital transducers called image-impedance connected interdigital transistors (IDTs) TC in the channels VE and VS respectively. The two image-impedance connected IDTs are placed, respectively, between the two input transducers TE and the two output transducers TS, and are series connected. Two consecutive cells of the filter have an input transducer and an output transducer in common. The filter also has reflecting gratings RE and RS at the ends of each of the channels VE and VS.
The image-impedance connected interdigital transducers TC contribute to a reduction in the losses due to the two-directional nature of the transducer. As shown in the appended FIG. 2, each image-impedance connected IDT has two facing, periodic, metallized combs, pe1 and pe2 with electrodes in the form of parallel, alternating fingers d1 and d2. The metallization spacing p between two adjacent fingers d1 and d2 is equal to the acoustic half wavelength .lambda./2, which corresponds to a finger-to-finger spacing equal to .lambda. each of the combs pe1 and pe2, as is known in transducers with interdigital electrodes. The width (a) of each of the fingers d1 and d2 is constant and equal to or smaller than .lambda./4. Thus the interval between the fingers is equal to p-a.
Under these conditions, the electrodes of the image-impedance connected IDT are equivalent to acoustic sources, the vibrations of which are added up in phase for frequencies corresponding to wavelengths close to twice the metallization spacing p. The cutoff frequency band of the transducer is then given by: EQU 2.pi.-2.DELTA..ltoreq..psi..sub.c .ltoreq.2.pi.+2.DELTA.radians
where .psi..sub.c is the electrical angle corresponding to the physical length 2p between two adjacent fingers in a comb, and .DELTA. is giving by .vertline..GAMMA..vertline.=sin .DELTA. where .GAMMA. is the measured coefficient of reflection of a finger.
To ensure that a filter such as this behaves satisfactorily in its pass band, the above-mentioned article recommends that the number of active fingers in each of the image-impedance connected IDTs should be substantially equal to 1.5/k.sup.2, where k.sup.2 designates the electromagnetic coupling coefficient of the substrate. When this condition is achieved, the radiation susceptance B(f) of the transducer remains close to zero in a frequency band with a relative width of the order of k.sup.2. This band shows very steep sides and practically no ripple.
However, the level of the minor lobes remains high, of the order of 20 dB, and the image-impedance connected IDTs have to be weighted to reduce the level of these lobes.
According to the prior art, two techniques are recommended to carry out this weighting. According to a first technique, called "apodization", and illustrated in FIG. 3a of the above-mentioned article, the length of the electrodes or fingers of the combs gradually diminishes on either side of the central active finger of the transducer. According to a second technique, called a "stairway" technique and illustrated in FIG. 3b of the above-mentioned article, the active fingers of uniform width have stepped configurations which are symmetrical, two by two, on either side of the rectilinear, central active finger of the transducer.
Furthermore, a third weighting technique may be considered according to the article by Clinton S. Hartmann, entitled "WEIGHTING INTERDIGITAL SURFACE WAVE TRANSDUCERS BE SELECTIVE WITHDRAWAL OF ELECTRODES", Ultrasonics Symposium Proceedings, IEEE, 1973, p. 423 to 426. This technique consists in selectively withdrawing electrodes or fingers in the first comb and/or the second comb of the transducer.
The present invention seeks to reduce the frequency bandwidth of an interdigital transducer designed, notably, to serve as an image-impedance connected IDT in a surface acoustic wave filter with low insertion losses.